For outdoor online diagnosis, the channels are subject to inevitable multipath scattering, which makes the above channel calculation unreliable.
Fortunately, for some applications, the IRS is installed in a fixed location (e.g. walls, ceilings, etc.) and one can adopt a base station as the RX which is also fixed.
Hence, both the IRS and the RX are located in an open area and the channel \(\mathbf {h}_\mathrm {RX}\)  between them can be assumed as a long-term stationary multipath-free channel [1]}, which can then be calculated using (REF ).
